Many investigators believe that the autonomic nervous system (ANS) may be involved in hyperactivity (HA) in children. The objectives of our studies are to investigate differences in autonomic functioning between HA and normal children by means of peripheral indicators, such as skin conductance, heart rate, and skin temperature, and to assess the effects of the drugs on both autonomic activity and on task performance which depends on attention such as reaction time. The test protocol includes recording the ANS variables during a rest period, presentation of a series of simple tones, and a reaction time task. The hypothesis that HA and normal children respond differently to stimulant drugs has been tested in a study of the effects of d-amphetamine on autonomic activity and attention in 6 to 13-year-old normal and HA boys. On the same protocol, male adults have been tested for age differences in drug effects. Studies have been done on the acute and chronic effects of caffeine on these measures in boys and in normal men. Results show that caffeine increases ANS activity in a manner similar to anxiety and that there are differences in placebo ANS activity and perhaps in caffeine effects among subjects according to their habitual caffeine usage. A new study of chronic (2-week) periods of caffeine or placebo intake is being carried out on children using an experimental design that should control for acute withdrawal effects.